High Resolution Intravascular Ultrasound Transducer Assembly Having A Flexible Substrate

ABSTRACT

An ultrasound transducer assembly of the present invention includes a flexible circuit to which an ultrasound transducer array and integrated circuitry are attached during fabrication of the ultrasound transducer assembly. The flexible circuit comprises a flexible substrate to which the integrated circuitry and transducer elements are attached while the flexible substrate is in a substantially flat shape. The flexible circuit further comprises electrically conductive lines that are deposited upon the flexible substrate. The electrically conductive lines transport electrical signals between the integrated circuitry and the transducer elements. After assembly, the flexible circuit is re-shapable into a final form such as, for example, a substantially cylindrical shape.

FIELD OF THE INVENTION

This invention relates to ultrasound imaging apparatuses placed within acavity to provide images thereof of the type described in Proudian etal. U.S. Pat. No. 4,917,097 and more specifically, to ultrasound imagingapparatuses and methods for fabricating such devices on a scale suchthat the transducer assembly portion of the imaging apparatus may beplaced within a vasculature in order to produce images of thevasculature.

BACKGROUND OF THE INVENTION

In the United States and many other countries, heart disease is aleading cause of death and disability. One particular kind of heartdisease is atherosclerosis, which involves the degeneration of the wallsand lumen of the arteries throughout the body. Scientific studies havedemonstrated the thickening of an arterial wall and eventualencroachment of the tissue into the lumen as fatty material builds uponthe vessel walls. The fatty material is known as “plaque.” As the plaquebuilds up and the lumen narrows, blood flow is restricted. If the arterynarrows too much, or if a blood clot forms at an injured plaque site(lesion), flow is severely reduced, or cut off and consequently themuscle that it supports may be injured or die due to a lack of oxygen.Atherosclerosis can occur throughout the human body, but it is most lifethreatening when it involves the coronary arteries which supply oxygento the heart. If blood flow to the heart is significantly reduced or cutoff, a myocardial infarction or “heart attack” often occurs. If nottreated in sufficient time, a heart attack often leads to death.

The medical profession relies upon a wide variety of tools to treatcoronary disease, ranging from drugs to open heart “bypass” surgery.Often, a lesion can be diagnosed and treated with minimal interventionthrough the use of catheter-based tools that are threaded into thecoronary arteries via the femoral artery in the groin. For example, onetreatment for lesions is a procedure known as percutaneous transluminalcoronary angioplasty (PTCA) whereby a catheter with an expandableballoon at its tip is threaded into the lesion and inflated. Theunderlying lesion is re-shaped, and hopefully, the lumen diameter isincreased to improve blood flow.

In recent years, a new technique has been developed for obtaininginformation about coronary vessels and to view the effects of therapy onthe form and structure of a site within a vessel rather then merelydetermining that blood is flowing through a vessel. The new technique,known as Intracoronary/Intravascular Ultrasound (ICUS/IVUS), employsvery small transducers arranged on the end of a catheter which provideelectronic transduced echo signals to an external imaging system inorder to produce a two or three-dimensional image of the lumen, thearterial tissue, and tissue surrounding the artery. These images aregenerated in substantially real time and provide images of superiorquality to the known x-ray imaging methods and apparatuses. Imagingtechniques have been developed to obtain detailed images of vessels andthe blood flowing through them. An example of such a method is the flowimaging method and apparatus described in O'Donnell et al. U.S. Pat. No.5,453,575, the teachings of which are expressly incorporated in theirentirety herein by reference. Other imaging methods and intravascularultrasound imaging applications would also benefit from enhanced imageresolution.

Known intravascular ultrasound transducer assemblies have limited imageresolution arising from the density of transducer elements that arearranged in an array upon a transducer assembly. Known intravasculartransducer array assemblies include thirty-two (32) transducer elementsarranged in a cylindrical array. While such transducer array assembliesprovide satisfactory resolution for producing images from within avasculature, image resolution may be improved by increasing the densityof the transducer elements in the transducer array.

However, reducing the size of the transducer array elements increasesthe diffraction of the ultrasound beam emitted by a transducer elementwhich, in turn, leads to decreased signal strength. For example, if thewidth of each of the currently utilized ferroelectric copolymertransducer elements is reduced by one-half so that sixty-four (64)transducer elements are arranged in a cylindrical array roughly the samesize as the thirty-two (32) transducer array, the strength of the signalproduced by the individual transducer elements in the sixty-four (64)element array falls below a level that is typically useful for providingan image of a blood vessel. More efficient transducer materials (havinga lower “insertion loss”) may be substituted for the ferroelectriccopolymer transducer material in order to provide a useful signal in anintravascular ultrasound transducer assembly having sixty-four (64)transducer elements in a cylindrical array. Such materials include leadzirconate titanate (PZT) and PZT composites which are normally used inexternal ultrasound apparatuses. However, PZT and PZT composites presenttheir own design and manufacturing limitations. These limitations arediscussed below.

In known ultrasound transducer assemblies, a thin glue layer bonds theferroelectric copolymer transducer material to the conductors of acarrier substrate. Due to the relative dielectric constants offerroelectric copolymer and epoxy, the ferroelectric copolymertransducer material is effectively capacitively coupled to theconductors without substantial signal losses when the glue layerthickness is on the order of 0.5 to 2.0 μm for a ferroelectric copolymerfilm that is 10-15 μm thick. This is a practically achievable glue layerthickness.

However, PZT and PZT composites have a relatively high dielectricconstant. Therefore capacitive coupling between the transducer materialand the conductors, without significant signal loss could occur onlywhen extremely thin glue layers are employed (e.g. 0.01 μm for a 10-15μm thick PZT transducer). This range of thicknesses for a glue layer isnot achievable in view of the current state of the art.

Transducer backing materials having relatively low acoustic impedanceimprove signal quality in transducer assemblies comprising PZT or PZTcomposites. The advantages of such backing materials are explained inEberle et al. U.S. Pat. No. 5,368,037 the teachings of which areexpressly incorporated in their entirety herein by reference. It is alsoimportant to select a matching layer for maximizing the acousticperformance of the PZT transducers by minimizing echoes arising from theultrasound assembly/blood-tissue interface.

Individual ferroelectric copolymer transducers need not be physicallyisolated from other transducers. However, PZT transducers must bephysically separated from other transducers in order to facilitateformation of the transducers into a cylinder and to provide desirableperformance of the transducers, such as minimization of acousticcrosstalk between neighboring elements. If the transducer elements arenot physically separated, then the emitted signal tends to conduct tothe adjacent transducer elements comprising PZT or PZT compositematerial.

Furthermore, the PZT and PZT composites are more brittle than theferroelectric copolymer transducer materials, and the transducerelements cannot be fabricated in a solid flat sheet and then re-shapedinto a cylindrical shape of the dimensions suitable for internalultrasound imaging.

The integrated circuitry of known ultrasound transducer probes aremounted upon a non-planar surface. (See, for example, the Proudian '097patent). The fabrication of circuitry on a non-planar surface addscomplexity to the processes for mounting the integrated circuitry andconnecting the circuitry to transmission lines connecting the integratedcircuitry to a transmission cable and to the transducer array.

Yet another limitation on designing and manufacturing higher densityultrasound transducer arrays for intravascular imaging is the density ofthe interconnection circuitry between the ultrasound transducer elementsand integrated circuits placed upon the ultrasound transducer assembly.Presently an interconnection density of about 0.002″ pitch betweenconnection points is achievable using state-of-the-art fabricationtechniques. However, in order to arrange sixty-four (64) elements in acylindrical array having a same general construction and size (i.e., 1.0mm) as the previously known 32 element array (e.g., the array disclosedin the Proudian et al. U.S. Pat. No. 4,917,097), the interconnectioncircuit density would have to increase. The resulting spacing of theinterconnection circuitry would have to be reduced to about 0.001″pitch. Such a circuit density is near the limits of current capabilitiesof the state of the art for reasonable cost of manufacturing.

SUMMARY OF THE INVENTION

It is a general object of the present invention to improve the imagequality provided by an ultrasound imaging apparatus over knownintravascular ultrasound imaging apparatuses.

It is another object of the present invention to decrease the per-unitcost for manufacturing ultrasound. transducer assemblies.

If is yet another object of the present invention to increase the yieldof manufactured ultrasound transducer assemblies.

It is a related object to increase image resolution by substantiallyincreasing the number of transducer elements in a transducer array whilesubstantially maintaining the size of the transducer array assembly.

The above mentioned and other objects are met in a new ultrasoundtransducer assembly and method for fabricating the ultrasound transducerassembly incorporating a flexible substrate. The ultrasound transducerassembly of the present invention includes a flexible circuit comprisinga flexible substrate and electrically conductive lines, deposited uponthe flexible substrate. An ultrasound transducer array and integratedcircuitry are attached during fabrication of the ultrasound transducerassembly while the flexible substrate is substantially planar (i.e.,flat). After assembly the electrically conductive lines transportelectrical signals between the integrated circuitry and the transducerelements.

The ultrasound transducer array comprises a set of ultrasound transducerelements. In an illustrative embodiment, the transducer elements arearranged in a cylindrical array. However, other transducer arrayarrangements are contemplated, such as linear, curved linear or phasedarray devices.

The integrated circuitry is housed within integrated circuit chips onthe ultrasound transducer assembly. The integrated circuitry is coupledvia a cable to an imaging computer which controls the transmission ofultrasound emission signals transmitted by the integrated circuitry tothe ultrasound transducer array elements. The imaging computer alsoconstructs images from electrical signals transmitted from theintegrated circuitry corresponding to ultrasound echoes received by thetransducer array elements.

The above described new method for fabricating an ultrasound catheterassembly retains a two-dimensional aspect to the early stages ofultrasound transducer assembly fabrication which will ultimately yield athree-dimensional, cylindrical device. Furthermore, the flexible circuitand method for fabricating an ultrasound transducer assembly accordingto the present invention facilitate the construction of individual,physically separate transducer elements in a transducer array.

BRIEF DESCRIPTION OF THE DRAWINGS

The appended claims set forth the features of the present invention withparticularity. The invention, together with its objects and advantages,may be best understood from the following detailed description taken inconjunction with the accompanying drawings of which:

FIG. 1 is a perspective view of the flat sub-assembly of an ultrasoundtransducer assembly incorporating a 64 element ultrasound transducerarray and integrated circuits mounted to a flexible circuit;

FIG. 2 is a schematic perspective view of the assembled ultrasoundtransducer assembly from the end containing the cable attachment pad;

FIG. 3 is a cross-section view of the ultrasound transducer assemblyillustrated in FIG. 2 sectioned along line 3-3 in the integrated circuitportion of the ultrasound transducer assembly;

FIG. 4 is a cross-section view of the ultrasound transducer assemblyillustrated in FIG. 2 sectioned along line 4-4 in the transducer portionof the ultrasound transducer assembly;

FIG. 5 is a longitudnal cross-section view of the ultrasound transducerassembly illustrated in FIG. 2 sectioned along line 5-5 and runningalong the length of the ultrasound transducer assembly;

FIG. 5 a is an enlarged view of the outer layers of the sectioned viewof the ultrasound transducer assembly illustratively depicted in FIG. 5;

FIG. 6 is an enlarged and more detailed view of the transducer region ofthe ultrasound transducer assembly illustratively depicted in FIG. 5;

FIG. 6 a is a further enlarged view of a portion of the transducerregion containing a cross-sectioned transducer;

FIG. 7 is a flowchart summarizing the steps for fabricating acylindrical ultrasound transducer assembly embodying the presentinvention;

FIG. 8 is a schematic drawing showing a longitudnal cross-section viewof a mandrel used to form a mold within which a partially assembledultrasound transducer assembly is drawn in order to re-shape the flat,partially assembled transducer assembly into a substantially cylindricalshape and to thereafter finish the ultrasound catheter assembly inaccordance with steps 114-120 of FIG. 7;

FIG. 9 is a schematic drawing of an illustrative example of anultrasound imaging system including an ultrasound transducer assemblyembodying the present invention and demonstrating the use of the deviceto image a coronary artery; and

FIG. 10 is an enlarged and partially sectioned view of a portion of thecoronary artery in FIG. 1 showing the ultrasound transducer assemblyincorporated within an ultrasound probe assembly located in a catheterproximal to a balloon and inserted within a coronary artery.

DETAILED DESCRIPTION OF THE DRAWINGS

Turning now to FIG. 1, a new ultrasound transducer assembly isillustratively depicted in its flat form in which it is assembled priorto forming the device into its final, cylindrical form. The ultrasoundtransducer assembly comprises a flex circuit 2, to which the otherillustrated components of the ultrasound transducer assembly areattached. The flex circuit 2 preferably comprises a flexible polyimidefilm layer (substrate) such as KAPTON™ by DuPont. However, othersuitable flexible and relatively strong materials, such as MYLAR(Registered trademark of E.I. DuPont) may comprise the film layer of theflex circuit 2. The flex circuit 2 further comprises metallicinterconnection circuitry formed from a malleable metal (such as gold)deposited by means of known sputtering, plating and etching techniquesemployed in the fabrication of microelectronic circuits upon a chromiumadhesion layer on a surface of the flex circuit 2.

The interconnection circuitry comprises conductor lines deposited uponthe surface of the flex circuit 2 between a set of five (5) integratedcircuit chips 6 and a set of sixty-four (64) transducer elements 8 madefrom PZT or PZT composites; between adjacent ones of the five (5)integrated circuit chips; and between the five (5) integrated circuitchips and a set of cable pads 10 for communicatively coupling theultrasound catheter to an image signal processor via a cable (notshown). The cable comprises, for example, seven (7) 43 AWG insulatedmagnet wires, spirally cabled and jacketed within a thin plastic sleeve.The connection of these seven cables to the integrated circuit chips 6and their function are explained in Proudian (deceased) et al. U.S. Pat.No. 4,917,097.

The width “W” of the individual conductor lines of the metalliccircuitry (on the order of one-thousandth of an inch) is relatively thinin comparison to the typical width of metallic circuitry deposited upona film or other flexible substrate. On the other, hand, the width of theindividual conductor lines is relatively large in comparison to thewidth of transmission lines in a typical integrated circuit. The layerthickness “T” of the conductor lines between the chips 6 and thetransducer elements 8 is preferably 2-5 μm. This selected magnitude forthe thickness and the width of the conductor lines enables the conductorlines to be sufficiently conductive while maintaining relativeflexibility and resiliency so that the conductor lines do not breakduring re-shaping of the flex circuit 2 into a cylindrical shape.

The thickness of the flex circuit 2 substrate is preferably on the orderof 12.5 μm to 25.0 μm. However, the thickness of the substrate isgenerally related to the degree of curvature in the final assembledtransducer assembly. The thin substrate of the flex circuit 2, as wellas the relative flexibility of the substrate material, enables the flexcircuit 2 to be wrapped into a generally cylindrical shape after theintegrated circuit chips 6 and the transducer elements 8 have beenmounted and formed and then attached to the metallic conductors of theflex circuit 2. Therefore, in other configurations, designs, andapplications requiring less or more substrate flexibility such as, forexample, the various embodiments shown in Eberle et al. U.S. Pat. No.5,368,037, the substrate thickness may be either greater or smaller thanthe above mentioned range. Thus, a flexible substrate thickness may beon the order of several (e.g. 5) microns to well over 100 microns (oreven greater)—depending upon the flexibility requirements of theparticular transducer assembly configuration.

The flex circuit is typically formed into a very small cylindrical shapein order to accommodate the space limitations of blood vessels. In suchinstances the range of diameters for the cylindrically shaped ultrasoundtransducer assembly is typically within the range of 0.5 mm. to 3.0 mm.However, it is contemplated that the diameter of the cylinder in anultrasound catheter for blood vessel imaging may be on the order of 0.3mm. to 5 mm. Furthermore, the flex circuit 2 may also be incorporatedinto larger cylindrical transducer assemblies or even transducerassemblies having alternative shapes including planar transducerassemblies where the flexibility requirements imposed upon the flexcircuit 2 are significantly relaxed. A production source of the flexcircuit 2 in accordance with the present invention is MetrigraphicsCorporation, 80 Concord Street, Wilmington, Mass. 01887.

The integrated circuit chips 6 are preferably of a type described in theProudian et al. U.S. Pat. No. 4,917,097 (incorporated herein byreference) and include the modifications to the integrated circuitsdescribed in the O'Donnell et al. U.S. Pat. No. 5,453,575 (alsoincorporated herein by reference). However, both simpler and morecomplex integrated circuits may be attached to the flex circuit 2embodying the present invention. Furthermore, the integrated circuitarrangement illustrated in FIG. 1 is intended to be illustrative. Thus,the present invention may be incorporated into a very wide variety ofintegrated circuit designs and arrangements are contemplated to fallwithin the scope of the invention.

Finally, the flex circuit 2 illustratively depicted in FIG. 1 includes atapered lead portion 11. As will be explained further below, thisportion of the flex circuit 2 provides a lead into a TEFLON (registeredtrademark of E.I. DuPont) mold when the flex circuit 2 and attachedcomponents are re-shaped into a cylindrical shape. Thereafter, the leadportion 11 is cut from the re-shaped flex circuit 2.

Turning to FIG. 2, an ultrasound transducer assembly is shown in are-shaped state. This shape is generally obtained by wrapping the flat,partially assembled ultrasound transducer assembly shown in FIG. 1 intoa cylindrical shape by means of a molding process described below. Atransducer portion 12 of the ultrasound transducer assembly containingthe transducer elements 8 is shaped in a cylinder for transmitting andreceiving ultrasound waves in a generally radial direction in aside-looking cylindrical transducer array arrangement. The transducerportion 12 on which the transducer elements 8 are placed mayalternatively be shaped or oriented in a manner different from thecylinder illustratively depicted in FIG. 2 in accordance withalternative fields of view such as side-fire planar arrays and forwardlooking planar or curved arrays.

The electronics portion 14 of the ultrasound transducer assembly is notconstrained to any particular shape. However, in the illustrativeexample the portions of the flex circuit 2 which support the integratedcircuits are relatively flat as a result of the electrical connectionsbetween the flex circuit and the integrated circuits. Thus the portionof the flex circuit 2 carrying five (5) integrated circuit chips 6 has apentagon cross-section when re-shaped (wrapped) into a cylinder. In analternative embodiment of the present invention, a re-shaped flexcircuit having four (4) integrated circuits has a rectangularcross-section. Other numbers of integrated circuits and resultingcross-sectional shapes are also contemplated.

FIG. 2 also shows the set of cable pads 10 on the flex circuit 2 whichextend from the portion of the flex circuit 2 supporting the integratedcircuit chips 6. A lumen 16 in the center of the ultrasound transducerassembly (within which a guidewire is threaded during the use of acatheter upon which the transducer assembly has been mounted) is definedby a lumen tube 18 made of a thin radiopaque material such asPlatinum/Iridium. The radiopaque material assists in locating theultrasound transducer assembly within the body during a medicalprocedure incorporating the use of the ultrasound transducer assembly.

Encapsulating epoxy 22 a and 22 b fills the spaces, respectively,between the integrated circuit chips 6 and a KAPTON tube 20, and aregion between the lumen tube 18 and the KAPTON tube 20 in the re-shapedultrasound transducer assembly illustrated in FIG. 2. The manner inwhich the encapsulating epoxy is applied during construction of theultrasound transducer device embodying the present invention isdescribed below in conjunction with FIG. 7 which summarizes the stepsfor fabricating such an ultrasound transducer assembly. The KAPTON tube20 helps to support the integrated circuits 6 during formation of theflex circuit 2 into the substantially cylindrical shaped deviceillustrated in FIG. 2. A more detailed description of the layers of thetransducer portion 12 and the electronics portion 14 of the ultrasoundtransducer assembly of the present invention is provided below.

Turning now to FIG. 3, a cross-section view is provided of theultrasound transducer assembly taken along line 3-3 and looking towardthe transducer portion 12 in FIG. 2. The outside of the electronicsportion 14 has a pentagon shape. The circular outline 26 represents theoutside of the transducer portion 12. The entire ultrasound transducerassembly is electrically shielded by a ground layer 28. The ground layer28 is encapsulated within a PARYLENE (registered trademark of UnionCarbide) coating 32.

Turning now to FIG. 4, a view is provided of a cross-section of theultrasound transducer assembly taken along line 4-4 and looking towardthe electronics portion 14 in FIG. 2. The five corners of the pentagonoutline comprising the electronics portion 14 are illustrated in thebackground of the cross-sectional view at line 4-4. The set ofsixty-four (64) transducer elements B are displayed in the foreground ofthis cross-sectional view of the transducer portion 12 of the ultrasoundtransducer assembly. A backing material 30 having a relatively lowacoustic impedance fills the space between the lumen tube 18 and thetransducer elements 8 as well as the gaps between adjacent ones of thesixty-four (64) transducer elements 8. The backing material 30 possessesthe ability to highly attenuate the ultrasound which is transmitted bythe transducer elements 8. The backing material 30 also providessufficient support for the transducer elements. The backing material 30must also cure in a sufficiently short period of time to meetmanufacturing needs. A number of known materials meeting the abovedescribed criteria for a good backing material will be known to thoseskilled in the art. An example of such a preferred backing materialcomprises a mixture of epoxy, hardener and phenolic microballoonsproviding high ultrasound signal attenuation and satisfactory supportfor the ultrasound transducer assembly.

Having generally described an ultrasound transducer assemblyincorporating the flex circuit in accordance with the present invention,the advantages provided by the flex circuit will now be described inconjunction with the illustrative embodiment. The flex circuit 2provides a number of advantages over prior ultrasound transducerassembly designs. The ground layer 28, deposited on the flex circuit 2while the flex circuit is in the flat state, provides an electricalshield for the relatively sensitive integrated circuit chips 6 andtransducer elements 8. The KAPTON substrate of the flex circuit 2provides acoustic matching for the PZT transducer elements 8, and thePARYLENE outer coating 32 of the ultrasound transducer assembly providesa second layer of acoustic matching as well as a final seal around thedevice.

The ease with which the flex circuit 2 may be re-shaped facilitatesmounting, formation and connection of the integrated circuit chips 6 andtransducer elements 8 while the flex circuit 2 is flat, and thenre-shaping the flex circuit 2 into its final state after the componentshave been mounted, formed and connected. The flex circuit 2 is heldwithin a frame for improved handling and positioning while the PZT andintegrated circuits are bonded to complete the circuits. The singlesheet of PZT or PZT composite transducer material is diced intosixty-four (64) discrete transducer elements by sawing or other knowncutting methods. After dicing the transducer sheet, kerfs exist betweenadjacent transducer elements while the flex circuit 2 is in the flatstate. After the integrated circuit chips 6 and transducer elements 8have been mounted, formed and connected, the flex circuit 2 is re-shapedinto its final, cylindrical shape by drawing the flex circuit 2 and themounted elements into a TEFLON mold (described further below).

Also, because the integrated circuits and transducer elements of theultrasound transducer assembly may be assembled while the flex circuit 2is in the flat state, the flex circuit 2 may be manufactured by batchprocessing techniques wherein transducer assemblies are assembledside-by-side in a multiple-stage assembly process. The flat, partiallyassembled transducer assemblies are then re-shaped and fabricationcompleted.

Furthermore, it is also possible to incorporate strain relief in thecatheter assembly at the set of cable pads 10. The strain reliefinvolves flexing of the catheter at the cable pads 10. Such flexingimproves the durability and the positionability of the assembledultrasound catheter within a patient.

Another important advantage provided by the flex circuit 2, is therelatively greater amount of surface area provided in which to lay outconnection circuitry between the integrated circuit chips 6 and thetransducer elements 8. In the illustrated embodiment of the presentinvention, the transducer array includes sixty-four (64) individualtransducer elements. This is twice the number of transducer elements ofthe transducer array described in the Proudian '097 patent. Doubling thenumber of transducer elements without increasing the circumference ofthe cylindrical transducer array doubles the density of the transducerelements. If the same circuit layout described in the Proudian '097 wasemployed for connecting the electronic components in the sixty-four (64)transducer element design, then the density of the connection circuitrybetween the integrated circuit chips 6 and the transducer elements 8must be doubled.

However, the flex circuit 2 occupies a relatively outer circumferenceof: (1) the transducer portion 12 in comparison to the transducerelements 8 and, (2) the electronics portion 14 in comparison to theintegrated circuit chips 6. The relatively outer circumference providessubstantially more area in which to lay out the connection circuitry forthe sixty-four (64) transducer element design in comparison to the areain which to lay out the connection circuitry in the designillustratively depicted in the Proudian '097 patent. As a result, eventhough the number of conductor lines between the integrated circuitchips 6 and the transducer elements 8 doubles, the density of theconductor lines is increased by only about fifty percent (50%) incomparison to the previous carrier design disclosed in the Proudian '097patent having a substantially same transducer assembly diameter.

Yet another advantage provided by the flex circuit 2 of the presentinvention is that the interconnection solder bumps, connecting themetallic pads of the integrated circuit chips 6 to matching pads on theflex circuit 2, are distributed over more of the chip 3 surface, so thesolder bumps only have to be slightly smaller than the previous designhaving only 32 transducer elements.

The integrated circuit chips 6 are preferably bonded to the flex circuit2 using known infrared alignment and heating methods. However, since theflex circuit 2 can be translucent, it is also possible to performalignment with less expensive optical methods which include viewing thealignment of the integrated circuit chips 6 with the connectioncircuitry deposited upon the substrate of the flex circuit 2 from theside of the flex circuit 2 opposite the surface to which the integratedcircuit chips 6 are to be bonded.

Turning now to FIGS. 5 and 5 a, a cross-sectional view and enlargedpartial cross-sectional view are provided of the ultrasound transducerassembly illustrated in FIG. 2 sectioned along line 5-5 and runningalong the length of the ultrasound transducer assembly embodying thepresent invention. The PARYLENE coating 32, approximately 5-20 μm inthickness, completely encapsulates the ultrasound transducer assembly.The PARYLENE coating 32 acts as an acoustic matching layer and protectsthe electronic components of the ultrasound transducer assembly.

The next layer, adjacent to the PARYLENE coating 32 is the ground layer28 which is on the order of 1-2 μm in thickness and provides electricalprotection for the sensitive circuits of the ultrasound transducerassembly. The next layer is a KAPTON substrate 33 of the flex circuit 2approximately 13 μm thick. Metallic conductor lines 34, approximately2-5 μm in thickness, are bonded to the KAPTON substrate 33 with achromium adhesion layer to form the flex circuit 2. While the metallicconductor lines 34 of the flex circuit 2 are illustrated as a solidlayer in FIG. 5, it will be appreciated by those skilled in the art thatthe metallic conductor lines 34 are fabricated from a solid layer (orlayers) of deposited metal using well known metal layer selectiveetching techniques such as masking or selective plating techniques. Inorder to minimize the acoustic affects of the conductive layers, themetal is on the order of 0.1 μm thick in the region of the transducer. Acable 35 of the type disclosed in the Proudian '097 patent is connectedto the cable pads 10 for carrying control and data signals transmittedbetween the ultrasound transducer assembly and a processing unit.

Next, a set of solder bumps such as solder bump 36 connect the contactsof the integrated circuit chips 6 to the metallic conductor lines 34 ofthe flex circuit 2. A two-part epoxy 38 bonds the integrated circuitchips 6 to the flex circuit 2. The integrated circuit chips 6 abut theKAPTON tube 20 having a diameter of approximately 0.030″ andapproximately 25 μm in thickness. The integrated circuit chips 6 areheld in place by the KAPTON tube 20 when the opposite side edges of theflex circuit 2 for the partially fabricated ultrasound transducerassembly are joined to form a cylinder.

FIG. 5 also shows the encapsulating epoxy 22 which fills the gapsbetween the integrated circuits and the space between the KAPTON tube 20and the lumen tube 18. The lumen tube 18 has a diameter of approximately0.024″ and is approximately 25 μm thick. A region at the transducerportion 12 of the ultrasound transducer assembly is filled by thebacking material 30 having a low acoustic impedance in order to inhibitringing in the ultrasound transducer assembly by absorbing ultrasoundwaves emitted by the transducer elements toward the lumen tube 18. Thetransducer portion 12 of the ultrasound transducer assembly of thepresent invention is described in greater detail below in conjunctionwith FIGS. 6 and 6 a.

Turning now to FIGS. 6 and 6 a (an enlarged portion of FIG. 6 providingadditional details regarding the structure of the transducer portion 12of the transducer assembly), the transducer elements 8 comprise a PZT orPZT composite 40 approximately 90 μm in thickness and, depending onfrequency, approximately 40 μm wide and 700 μm long. Each transducerelement includes a Cr/Au ground layer 42, approximately 0.1 μm inthickness, connected via a silver epoxy bridge 44 to the ground layer28. Each transducer element includes a Cr/Au electrode layer 46,approximately 0.1 μm in thickness. The Cr/Au electrode layer 46 isdirectly bonded to the PZT or PZT composite 40. The electrode layer 46of each transducer element is electrically connected to a correspondingelectrode 47 by means of several contacts such as contacts 48. Theseveral contacts for a single transducer are used for purposes ofredundancy and reliability and to act as a spacer of constant thicknessbetween the electrode 47 and the PZT composite 40 of a transducerelement. Each electrode such as electrode 47 is connected to one of themetallic conductor lines 34 of the flex circuit 2. The thickness of theelectrode 47 is less than the thickness of the metallic conductor lines34 in order to enhance acoustic response of the transducer elements 8.The corresponding conductor line couples the transducer element to anI/O channel of one of the integrated circuit chips 6. A two-part epoxy50, approximately 2-5 μm in thickness, fills the gaps between theelectrode layer 46 and the flex circuit 2 (comprising the substrate 33and metal layers 34 and 28, and can also be selected to act as anacoustic matching layer.

Finally, as will be explained further below in conjunction with steps112 and 118 in FIG. 7, the backing material 30 is applied in twoseparate steps. At step 112, a cylinder 30 a of backing material ismolded directly upon the lumen tube 18. During step 118, the remainingportions 30 b and 30 c are injected to complete the backing materialportion. It is further noted that while the barrier between theencapsulating epoxy 22 and the backing material 30 is shown as a flatplane in the figures, this barrier is not so precise—especially withrespect to the portions 30 b and 30 c which are applied by injecting thebacking material through the kerfs between adjacent transducers.

Turning now to FIG. 7, the steps are summarized for fabricating theabove-described ultrasound transducer assembly embodying the presentinvention. It will be appreciated by those skilled in the art that thesteps may be modified in alternative embodiments of the invention.

At step 100, the flex circuit 2 is formed by depositing layers ofconductive materials such as Chromium/Gold (Cr/Au) on a surface of theKAPTON substrate 33. Chromium is first deposited as a thin adhesionlayer, typically 50-100 Angstroms thick, followed by the gold conductinglayer, typically 2-5 μm thick. Using well known etching techniques,portions of the Cr/Au layer are removed from the surface of the KAPTONsubstrate 33 in order to form the metallic conductor lines 34 of theflex circuit 2. The ground layer 28, also made up of Cr/Au is depositedon the other surface of the flex circuit 2. The ground layer 28 istypically kept thin in order to minimize its effects on the acousticperformance of the transducer.

During the formation of the conductor lines, the gold bumps, used tomake contact between the PZT transducer conductive surface and theconductor lines on the flex circuit, are formed on the flex circuit 2.Also, in the transducer region, as previously stated, the Cr/Au layer istypically kept thin in order to allow a stand-off for the adhesionlayer, and so that the metal has a minimum effect on the acousticperformance of the transducer. This can be achieved by performing asecondary metallization stage after the formation of the conductinglines and the gold bumps.

In a separate and independent procedure with respect to theabove-described step for fabricating the flex circuit 2, at step 102metal layers 42 and 46 are deposited on the PZT or PZT composite 40 toform a transducer sheet. Next, at step 104, the metallized PZT or PZTcomposite 40 is bonded under pressure to the flex circuit 2 using atwo-part epoxy 50, and cured overnight. The pressure exerted duringbonding reduces the thickness of the two-part epoxy 50 to a thickness ofapproximately 2-5 μm, depending on the chosen thickness of the goldbumps. The very thin layer of two-part epoxy 50 provides good adhesionof the metallized PZT or PZT composite to the flex circuit 2 withoutsignificantly affecting the acoustic performance of the transducerelements 8. During exertion of pressure during step 104, a portion ofthe two-part epoxy 50 squeezes out from between the flex circuit 2 andthe transducer sheet from which the transducer elements 8 will beformed. That portion of the two-part epoxy 50 forms a fillet at each endof the bonded transducer sheet (See FIG. 6). The fillets of the two-partepoxy 50 provide additional support for the transducer elements 8 duringsawing of the PZT or PZT composite into separate transducer elements.Additional two-part epoxy 50 may be added around the PZT to make thefillet more uniform.

At step 106, after the two-part epoxy 50 is cured and before the PZT orPZT composite 40 is separated into 64 discrete transducer elements, thefirst part of the silver epoxy bridges, such as silver epoxy bridge 44,is formed. The silver epoxy bridges conductively connect the groundlayer (such as ground layer 42) of the transducer elements 8 to theground layer 28 on the opposite surface of the flex circuit 2. Thesilver epoxy bridges such as silver epoxy bridge 44 are formed in twoseparate steps. During step 106, the majority of each of the silverepoxy bridges is formed by depositing silver epoxy upon the ground layerof the transducer elements 8 such as ground layer 42, the fillet formedon the side of the transducer material by the two-part epoxy 50, and theKAPTON substrate 33. The silver epoxy bridges are completed during alater stage of the fabrication process by filling vias formed in theKAPTON substrate 33 of the flex circuit 2 with silver epoxy material.These vias may be formed by well known “through-hole” plating techniquesduring the formation of the flex circuit 2, but can also be formed bysimply cutting a flap in the relatively thin flex circuit 2 material andbending the flap inward towards the center of the cylinder when thefabricated flex circuit and components are re-shaped. Thereafter, thesilver epoxy bridge 44 is completed by adding the conductive material tothe via on the inside of the cylinder with no additional profile to thefinished device.

In order to obtain good performance of the elements and to facilitatere-shaping the flex circuit 2 into a cylinder after the integratedcircuit chips 6 and transducer elements 8 have been attached, thetransducer elements 8 are physically separated during step 108. Dicingis accomplished by means of a well known high precision, high speed discsawing apparatus, such as those used for sawing silicon wafers. It isdesirable to make the saw kerfs (i.e., the spaces between the adjacenttransducer elements) on the order of 15-25 μm when the flex circuit isre-shaped into a cylindrical shape. Such separation dimensions areachieved by known high precision saw blades having a thickness of 10-15μm.

After the two part epoxy 50 is fully cured, the flex circuit 2 isfixtured in order to facilitate dicing of the transducer material intosixty-four (64) discrete elements. The flex circuit 2 is fixtured byplacing the flex circuit 2 onto a vacuum chuck (of well known design forprecision dicing of very small objects such as semiconductor wafers)which is raised by 50-200 μm in the region of the transducer elements 8in order to enable a saw blade to penetrate the flex circuit 2 in theregion of the transducer elements 8 without affecting the integratedcircuit region. The saw height is carefully controlled so that the cutextends completely through the PZT or PZT composite 40 and partiallyinto the KAPTON substrate 33 of the flex circuit 2 by a few microns. Inorder to further reduce the conduction of ultrasound to adjacenttransducer elements, the cut between adjacent transducer elements mayextend further into the flex circuit 2. The resulting transducer elementpitch (width) is on the order of 50 μm. In alternative embodiments thiscut may extend all the way through the flex circuit 2 in order toprovide full physical separation of the transducer elements.

Alternatively the separation of transducer elements may possibly be donewith a laser. However, a drawback of using a laser to dice thetransducer material is that the laser energy may depolarize the PZT orPZT composite 40. It is difficult to polarize the separated PZTtransducer elements, and therefore the sawing method is presentlypreferred.

After the PZT or PZT composite 40 has been sawed into discretetransducer elements and cleaned of dust arising from the sawing of thePZT or PZT composite 40, at step 110 the integrated circuit chips 6 areflip-chip bonded in a known manner to the flex circuit 2 using pressureand heat to melt the solder bumps such as solder bump 36. The integratedcircuit chips 6 are aligned by means of either infrared or visible lightalignment techniques so that the Indium solder bumps on the integratedcircuits 6 align with the pads on the flex circuit 2. These alignmentmethods are well known to those skilled in the art. The partiallyassembled ultrasound transducer assembly is now ready to be formed intoa substantially cylindrical shape as shown in FIGS. 2, 3 and 4.

Before re-shaping the flat flex circuit 2 (as shown in FIG. 1) into acylindrical shape around the lumen tube 18, at step 112 backing material30 is formed into a cylindrical shape around the lumen tube 18 using amold. Pre-forming the backing material 30 onto the lumen tube 18, ratherthan forming the flex circuit 2 and backfilling the cylinder withbacking material, helps to ensure concentricity of the transducerportion 12 of the assembled ultrasound transducer device around thelumen tube 18 and facilitates precise forming of the backing materialportion of the ultrasound transducer apparatus embodying the presentinvention.

At step 114, the lumen tube 18, backing material 30, and the partiallyassembled flex circuit 2 are carefully drawn into a preformed TEFLONmold having very precise dimensions. The TEFLON mold is formed by heatshrinking TEFLON tubing over a precision machined mandrel (as shown inFIG. 8 and described below). The heat shrinkable TEFLON tubing is cutaway and discarded after fabrication of the ultrasound transducerassembly is complete. As a result, distortion of a mold through multipleuses of the same mold to complete fabrication of several ultrasoundtransducer assemblies is not a problem, and there is no clean up of themold required.

The TEFLON molds incorporate a gentle lead-in taper enabling the sidesof the flex circuit 2 to be carefully aligned, and the gap between thefirst and last elements to be adjusted, as the flex circuit 2 is pulledinto the mold. In the region of the transducer, the mold is held to adiametric precision of 2-3 μm. Since the flex circuit 2 dimensions areformed with precision optical techniques, the dimensions are repeatableto less than 1 μm, the gap between the first and last elements (on theouter edges of the flat flex circuit 2) can be repeatable and similar tothe kerf width between adjacent elements.

While the flex circuit 2 is drawn into the TEFLON mold during step 114,the KAPTON tube 20 is inserted into the TEFLON mold between theintegrated circuits 6 (resting against the outer surface of the KAPTONtube 20) and the lumen tube 18 (on the inside). The KAPTON tube 20causes the flex circuit 2 to take on a pentagonal cross-section in theelectronics portion 14 of the ultrasound transducer assembly by applyingan outward radial force upon the integrated circuits 6. The outwardradial force exerted by the KAPTON tube 20 upon the integrated circuits6 causes the flex circuit 2 to press against the TEFLON mold at fiveplaces within the cylindrical shape of the TEFLON mold.

A TEFLON bead is placed within the lumen tube 18 in order to preventfilling of the lumen 16 during the steps described below for completingfabrication of the ultrasound transducer assembly. While in the mold,the partially assembled ultrasound transducer assembly is accessed fromboth open ends of the mold in order to complete the fabrication of theultrasound transducer assembly.

Next, at step 116 the silver epoxy bridges (e.g., bridge 44) connectingthe ground layer of each of the discrete transducers (e.g., ground layer42) to the ground layer 28 are completed. The connection is completed byinjecting silver epoxy into the vias such as via 45 in the KAPTONsubstrate 33. The bridges are completed by filling the vias after theflex circuit 2 has been re-shaped into a cylinder. However, inalternative fabrication methods, the vias are filled while the flexcircuit 2 is still in its flat state as shown in FIG. 1.

The lumen tube 18 is also connected to the ground layer 28 at the distalend of the ultrasound transducer assembly. Alternatively, the lumen tube18 and ground layer 28 are connected to electrical ground wire of thecable 35 at the proximal end of the ultrasound transducer assembly.

After the ground layer 42 of the transducers is connected to the groundplane 28 and the silver epoxy bridge 44 is cured, at step 118 additionalbacking material 30 is injected into the distal end of the ultrasoundtransducer assembly in order to fill the kerfs between transducerelements and any gaps between the preformed portion of the backingmaterial 30 and the transducer elements 8. This ensures that there areno air gaps in the region of the backing material 30 since air gapsdegrade the performance of the ultrasound transducer assembly anddegrade the mechanical integrity of the device.

At step 120, after the part of the backing material 30 added during step118 cures, the encapsulating epoxy 22 is injected into the electronicsportion 14 of the ultrasound transducer assembly at the end housing theintegrated circuit chips 6.

At step 122, after the encapsulating epoxy 22 and backing material 30are cured, the ultrasound transducer assembly is removed from the moldby either pushing the device out of the mold or carefully cutting theTEFLON mold and peeling it from the ultrasound transducer assembly. TheTEFLON bead is removed from the lumen tube 18. Stray encapsulating epoxyor backing material is removed from the device.

Next, at step 124 the device is covered with the PARYLENE coating 32.The thickness of the PARYLENE coating 32 is typically 5-20 μm. ThePARYLENE coating 32 protects the electronic circuitry and transducers ofthe ultrasound transducer assembly and provides a secondary matchinglayer for the transducer elements 8. The individual conductors of thecable 35 are bonded to the cable pads 10.

Having described one method for fabricating an ultrasound transducerassembly incorporating the flex circuit 2, it is noted that the order ofthe steps is not necessarily important. For example, while it ispreferred to attach the integrated circuits 6 to the flex circuit 2after the transducers 6 have been bonded to the flex circuit 2, such anorder for assembling the ultrasound transducer assembly is notessential. Similarly, it will be appreciated by those skilled in the artthat the order of other steps in the described method for fabricating anultrasound transducer assembly can be re-arranged without departing fromthe spirit of the present invention.

Turning briefly to FIG. 8, a longitudnal cross-section view is providedof the mandrel previously mentioned in connection with the descriptionof step 114 above. The mandrel enables a TEFLON tube to be re-formedinto a mold (shown generally by a ghost outline) having very preciseinside dimensions by heat shrinking the TEFLON tube onto the mandrel.The TEFLON mold is thereafter used to re-shape the partially assembledultrasound transducer assembly during step 114. While precise dimensionsand tolerances are provided on the drawing, they are not intended to belimiting since they are associated with a particular size and shape foran ultrasound transducer assembly embodying the present invention.

The mandrel and resulting inside surface of the TEFLON mold generallydisplay certain characteristics. First, the mandrel incorporates a taperfrom a maximum diameter at the end where the flex circuit enters themold to a minimum diameter at the portion of the mold corresponding tothe transducer portion of the ultrasound transducer assembly. This firstcharacteristic facilitates drawing the flex circuit into the mold.

Second, the mold has a region of constant diameter at the region wherethe integrated circuit portion will be formed during step 114. Thisdiameter is slightly greater than the diameter of the transducer regionof the mold where the diameter of the inside surface is precisely formedinto a cylinder to ensure proper mating of the two sides of the flexcircuit when the flat, partially assembled transducer assembly isre-shaped into a cylindrical transducer assembly. The greater diameterin the integrated circuit region accommodates the points of the pentagoncross-section created by the integrated circuit chips 6 when the flatflex circuit is re-shaped into a cylinder.

Finally, a second taper region is provided between the integratedcircuit and transducer portions of the mold in order to provide a smoothtransition from the differing diameters of the two portions.

The above description of the invention has focused primarily upon thestructure, materials and steps for constructing an ultrasound transducerassembly embodying the present invention. Turning now to FIGS. 9 and 10,an illustrative example of the typical environment and application of anultrasound device embodying the present invention is provided. Referringto FIGS. 9 and 10, a buildup of fatty material or plaque 70 in acoronary artery 72 of a heart 74 may be treated in certain situations byinserting a balloon 76, in a deflated state, into the artery via acatheter assembly 78. As illustrated in FIG. 9, the catheter assembly 78is a three-part assembly, having a guide wire 80, a guide catheter 78 afor threading through the large arteries such as the aorta 82 and asmaller diameter catheter 78 b that fits inside the guide catheter 78 a.After a surgeon directs the guide catheter 78 a and the guide wire 80through a large artery leading via the aorta 82 to the coronaryarteries, the smaller catheter 78 b is inserted. At the beginning of thecoronary artery 72 that is partially blocked by the plaque 70, the guidewire 80 is first extended into the artery, followed by catheter 78 b,which includes the balloon 76 at its tip.

Once the balloon 76 has entered the coronary artery 72, as in FIG. 10,an ultrasonic imaging device including a probe assembly 84 housed withinthe proximal sleeve 86 of the balloon 76 provides a surgeon with across-sectional view of the artery on a video display 88. In theillustrated embodiment of the invention, the transducers emit 20 MHzultrasound excitation waveforms. However, other suitable excitationwaveform frequencies would be known to those skilled in the art. Thetransducers of the probe assembly 84 receive the reflected ultrasonicwaveforms and convert the ultrasound echoes into echo waveforms. Theamplified echo waveforms from the probe assembly 84, indicative ofreflected ultrasonic waves, are transferred along a microcable 90 to asignal processor 92 located outside the patient. The catheter 78 b endsin a three-part junction 94 of conventional construction that couplesthe catheter to an inflation source 96, a guide wire lumen and thesignal processor 92. The inflation and guide wire ports 94 a and 94 b,respectively, are of conventional PTCA catheter construction. The thirdport 94 c provides a path for the cable 90 to connect with the signalprocessor 92 and video display 88 via an electronic connector 98.

It should be noted that the present invention can be incorporated into awide variety of ultrasound imaging catheter assemblies. For example, thepresent invention may be incorporated in a probe assembly mounted upon adiagnostic catheter that does not include a balloon. In addition, theprobe assembly may also be mounted in the manner taught in Proudian etal. U.S. Pat. No. 4,917,097 and Eberle et al. U.S. Pat. No. 5,167,233,the teachings of which are explicitly incorporated, in all respects,herein by reference. These are only examples of various mountingconfigurations. Other configurations would be known to those skilled inthe area of catheter design.

Furthermore, the preferred ultrasound transducer assembly embodying thepresent invention is on the order of a fraction of a millimeter toseveral millimeters in order to fit within the relatively smallcross-section of blood vessels. However, the structure and method formanufacturing an ultrasound transducer assembly in accordance withpresent invention may be incorporated within larger ultrasound devicessuch as those used for lower gastrointestinal examinations.

Illustrative embodiments of the present invention have been provided.However, the scope of the present invention is intended to include,without limitation, any other modifications to the described ultrasoundtransducer device and methods of producing the device falling within thefullest legal scope of the present invention in view of the descriptionof the invention and/or various preferred and alternative embodimentsdescribed herein. The intent is to cover all alternatives, modificationsand equivalents included within the spirit and scope of the invention asdefined by the appended claims.

1. An ultrasound transducer assembly for facilitating providing imagesfrom within a cavity, the ultrasound transducer assembly comprising: anultrasound transducer array comprising a set of ultrasound transducerelements; integrated circuitry; and a flexible circuit to which theultrasound transducer array and integrated circuitry are attached duringfabrication of the ultrasound transducer assembly, the flexible circuitcomprising: a flexible substrate, providing a re-shapable platform, towhich the integrated circuitry and transducer elements are attached; andelectrically conductive lines deposited upon the flexible substrate fortransporting electrical signals between the integrated circuitry and thetransducer elements.
 2. The ultrasound transducer assembly of claim 1wherein the ultrasound transducer array is substantially cylindrical inshape.
 3. The ultrasound transducer assembly of claim 2, having suitabledimensions for providing images of a blood vessel from within avasculature, and wherein the diameter of the substantially cylindricalultrasound transducer assembly is on the order of 0.3 to 5.0millimeters.
 4. The ultrasound transducer assembly of claim 2 whereinthe flexible circuit is substantially cylindrical in shape and occupiesa relatively outer position than the integrated circuitry with respectto a central axis of the cylindrical ultrasound transducer assembly. 5.The ultrasound transducer assembly of claim 2 wherein the electricallyconductive lines deposited upon the flex circuit occupy a relativelyouter position in relation to the transducer elements, with respect to acentral axis of the ultrasound transducer assembly in a transducerportion of the ultrasound transducer assembly.
 6. The ultrasoundtransducer assembly of claim 2 wherein the electrically conductive linesdeposited upon the flex circuit occupy a relatively outer position inrelation to the integrated circuitry, with respect to a central axis ofthe ultrasound transducer assembly in an electronics portion of theultrasound transducer assembly.
 7. The transducer assembly of claim 1wherein the substrate comprises a polyimide.
 8. The transducer assemblyof claim 1 wherein the substrate thickness is substantially within therange of 5 microns to 100 microns.
 9. The transducer assembly of claim 1wherein the layer thickness of the electrically conductive lines issubstantially in the range of 2-5 microns.
 10. The ultrasound transducerassembly of claim 1 wherein the ultrasound transducer elements comprisePZT material.
 11. The ultrasound transducer assembly of claim 10 whereinthe PZT material is a PZT composite.
 12. The ultrasound transducerassembly of claim 10 wherein the PZT material is directly bonded toconductive material comprising the electrode coupled to a communicationchannel in the integrated circuitry.
 13. The ultrasound transducerassembly of claim 1 wherein the ultrasound transducer elements compriseat least 32 transducer elements.
 14. The ultrasound transducer assemblyof claim 1 wherein the ultrasound transducer elements comprise at least48 transducer elements.
 15. The ultrasound transducer assembly of claim1 wherein the ultrasound transducer elements comprise at least 64transducer elements.
 16. In an ultrasound transducer assembly, aflexible circuit to which integrated circuitry and transducer elementsare attached, the flexible circuit comprising: a flexible substrate towhich the integrated circuitry and transducer elements are attachedprior to re-shaping the flexible substrate from a substantially planarshape into a non-planar shape; and electrically conductive lines,deposited upon the flexible substrate while the flexible substrate is inthe substantially planar shape, for transporting electrical signalsbetween the integrated circuitry and the transducer elements.
 17. Amethod for fabricating an ultrasound transducer assembly comprising aflexible circuit, integrated circuitry, and a set of transducer elementsfor facilitating providing images of a blood vessel from within avasculature, the method comprising the steps: fabricating the flexiblecircuit comprising a flexible substrate and a set of electricallyconductive lines formed upon the flexible substrate; constructing theset of transducer elements upon the flexible circuit and attaching theintegrated circuitry to the flexible circuit while the flexible circuitis in a substantially flat shape; and re-shaping the flexible circuitinto a substantially non-flat shape after the step of constructing a setof transducer elements and attaching the integrated circuitry.
 18. Themethod of claim 17 wherein the set of transducer elements comprise PZTmaterial.
 19. The method of claim 18 wherein the step of constructing aset of transducer elements upon the flexible circuit comprises bondingconductive material directly to the PZT material.
 20. The method ofclaim 19 wherein the conductive material bonded directly to the PZTmaterial forms a set of excitation electrodes coupled to the integratedcircuitry via the set of electrically conductive lines.
 21. The methodof claim 20 wherein the conductive material further comprises groundelectrodes.
 22. The method of claim 18 wherein the step of constructingthe set of transducer elements upon the flexible circuit comprisesdicing a metallized sheet of PZT material into at least 32 transducerelements.
 23. The method of claim 18 wherein the step of constructing aset of transducer elements upon the flexible circuit comprises dicing ametallized sheet of PZT material into at least 48 transducer elements.24. The method of claim 18 wherein the step of constructing a set oftransducer elements upon the flexible circuit comprises dicing ametallized sheet of PZT material into at least 64 transducer elements.25. The method of claim 17 wherein the re-shaping step comprises shapingthe flexible circuit into a substantially cylindrical shape.
 26. Themethod of claim 25 wherein the flexible circuit occupies a relativelyouter position than the integrated circuitry with respect to a centralaxis of the ultrasound transducer assembly after the re-shaping step.27. The method of claim 25 wherein electrodes for the transducerelements coupled to the integrated circuitry occupy a relatively outerposition than the ground electrodes for the transducer elements withrespect to a central axis of the ultrasound transducer assembly afterthe re-shaping step.
 28. An ultrasonic transducer assembly mounted to adistal end of a catheter for providing images within a vascular system,the assembly comprising: an array of transducers for transmitting andreceiving ultrasonic signals; electronic circuitry for controlling thetransmission and reception of the ultrasonic signals by the array oftransducers; a flexible substrate supporting the array of transducers,the electronic circuitry, and electrically conductive paths thattransport electrical signals between the electronic circuitry and thearray of transducers; a first configuration of the flexible substrate inwhich the array of transducers and the electronic circuitry are attachedto the substrate; and, a second configuration of the flexible substratein which the substrate, array of transducers and electronic circuitryare attached to the distal end of the catheter.
 29. An ultrasonictransducer assembly mounted to a distal end of a catheter providingimages within a vascular system made by the following process: printingelectrically conductive paths on a flexible substrate in a substantiallyflat configuration; attaching to the flexible substrate in thesubstantially flat configuration an array of transducers fortransmitting and receiving ultrasonic signals and electronic circuitryfor controlling the transmission and reception of the ultrasonic signalsby the array of transducers; bending the flexible substrate in asubstantially annular configuration; and, securing to the distal end ofthe catheter the substrate in the substantially annular configurationwith the attached array of transducers and electronic circuitry.